The present application relates generally to the field of automated apparatus for handling electronic circuit components, and, more particularly, to automated apparatus for use in the art of burning-in and testing circuit components prior to their distribution and use. Still more particularly, the present invention is directed to an automated system wherein electronic integrated circuit (IC) packages are placed in the burn-in system in their storage containers, automatically unloaded from their containers and inserted into sockets on printed circuit (PC) boards, subjected to burn-in and, in some cases, testing in an environmentally controlled chamber; automatically removed from the PC boards, and loaded into storage devices which indicate how the IC's performed during the burn-in and testing. The entire process, from the initial placement of the IC's into the system to the removal of the tested and graded IC's from the system, is completely automated and therefore capable of "hands-off" operation.
According to present practices, IC packages are mass-produced and installed in electronic circuits within highly sophisticated, complex and costly equipment. As with many mass-produced products, IC packages are prone to failure, in many cases at the beginning of operation. The complexity of the equipment within which such packages are installed makes post-installation failures highly undesirable. For example, when equipment reaches the final inspection stage of production, before failures are detected, the high level skills required for testing and repair add a significant cost to production expenses. Even more significantly, when the product has been installed in the field and a service technician must make warranty repairs, the costs thereby incurred can have a significant effect on profitability. As a result, manufacturers of electronic equipment are demanding ever greater quality and dependability in commercial grade IC packages.
Quality and dependability are enhanced substantially by detection of those IC packages likely to fail in the first few hours of operation, prior to installation of the packages in electronic equipment. One of the methods for exposing flawed IC packages is referred to as "burn-in." According to burn-in techniques, IC packages are stressed within their physical and electrical limits prior to installation, whereby those packages likely to become early failures in completed equipment can be discovered.
Burn-in involves placing a large number of IC packages on one or more printed circuit boards ("burn=in boards"); placing the burn-in boards with the packages mounted thereon in a chamber whose environment, particularly temperature, is controllable; applying direct current (dc) biases to each package on each board in such a manner as to forward and reverse bias as many of the package's junctions as possible, and/or actively clocking each package to its maximum rated conditions, such application of dc biases and clock signals being accomplished substantially simultaneously to each package; removing the burn-in boards from the chamber after the IC packages have been subjected to the environmental condition of the chamber and the biases and clock signals for a designated period of time; and removing the IC packages from the burn-in boards.
The burn-in process may encompass one of five general combinations of events. The simplest form of burn-in exposes the devices under test to a specific temperature without supplying power or clocking signals to the devices. In a second form, the devices are exposed to a particular temperature and supplied with power but not with clocking signals. In a third form, the devices are raised to a particular temperature and are exercised by the addition of both power and clocking signals, but no effort is made to monitor or evaluate performance. This third is probably the most popular form of burn-in at the present time. In a fourth form, which is gaining popularity at present, the devices are powered and clocked at a particular temperature, and the burn-in board input signals are monitored to insure that a short circuit or an open circuit at one device does not defeat the exercising of other devices. Finally, a fifth form of burn-in, which is popular especially for memory devices, involves monitoring and functional testing of devices that are powered and clocked at a particular temperature by monitoring output signals received from the memory devices. As used herein, the phrase "burn-in" or "burn-in process," unless otherwise specified, refers to any one of the five forms of burn-in described above.
A second method for improving quality control of the IC package subsequent to burn-in is to verify that the IC package functions according to its minimum rated specifications. Typically, each IC package is tested across a broad range of parameters and graded in quality according to its performance. Thereafter, the IC packages may be sorted into groups according to the predetermined performance grades.
The IC packages can be electrically tested either within or outside the burn-in chamber, depending on the sophistication of the particular chamber, by applying a room temperature test of critical dc parameters; for example, input currents and thresholds, output voltages and currents, and, in the case of digital components, by making a functional test to verify truth table performance. In this way, the packages that fail during burn-in are detected and segregated from those that do not fail. Because the packages that do not fail during the burn-in process have withstood substantial stress, such IC packages possess a high degree of dependability and can be installed in highly complex equipment with reasonable confidence that such IC packages will not fail prematurely.
The burn-in and testing processes, however, although successful in reducing the expense of troubleshooting failed electronic equipment, are not themselves without expense. Substantial capital expenditures are necessary to purchase or construct burn-in chambers, burn-in boards, and test equipment. Personnel must be employed and trained to operate the equipment and to monitor the time-consuming processes. In some cases entire businesses have been built around performance of the burn-in and testing processes. Use of the processes and, consequently, the success of a business that provides such services, is dependent upon the cost effectiveness of burning-in and testing the IC package vis-a-vis not burning-in or testing the IC packages but instead replacing those IC packages that fail after installation and use in the field.
One means for improving the cost effectiveness of the burn-in process is a reduction in labor cost. There is labor cost associated with almost all steps in the burn-in process. Consequently, efforts have been undertaken to automate certain stages of the process such as the loading of IC packages into burn-in board sockets and unloading and sorting the same (for example, see the commonly invented and assigned U.S. Pat. No. 4,567,652, entitled "Burn-In Board Loader", and U.S. Pat. No. 4,584,764, entitled "Burn-In Board Unloader and Package Sorter"). The savings in labor cost resulting from automating certain stages of the burn-in process can be substantial.
Increasing the cost effectiveness of the burn-in and testing processes can also be done by increasing the throughput of the IC packages. Since the burn-in process takes a great deal of time (from 6 to 160 hours), it is particularly advantageous to increase the number of IC packages which can be placed in the burn-in chambers. Another way to increase the cost effectiveness of the burn-in and testing process is to utilize equipment which is less subject to deterioration so that repair and replacement costs may be kept to a minimum. In addition, the reliability of the burn-in and testing process is improved when equipment is less prone to give inaccurate results due to deterioration.
A savings in the cost of labor is not the sole justification for automating the various stages of the burn-in board process. Increased reliability arises from the elimination of human error and the reduction of contamination when automated "hands-off" operation is present.
In an effort to realize the full benefits of automation, the focus in electronic factories has turned to automating entire processes, instead of merely providing islands of automation. Problems immediately arise, however, in how to integrate smoothly all components in a process. This problem is intensified in the burn-in process primarily due to the uniqueness of the process. Unlike the normal process of loading IC packages onto PC boards, the burn-in system utilizes a burn-in board with a uniform arrangement of sockets across the entire board; whereas a typical PC board, not designed for burn-in use is characterized by a random arrangement of components positioned across a board designed for a particular application.
A burn-in process, unlike the normal process of loading IC packages onto PC boards, requires that the IC packages be unloaded from the burn-in board after the components have been subjected to burn-in and testing. This creates a myriad of difficulties such as unloading the IC packages without damaging them or the burn-in board; identifying which IC packages passed and failed the burn-in and testing process; categorizing those IC packages according to performance; precisely identifying burn-in board and socket locations; and identifying what type of IC packages are being tested presently so that the IC packages may be handled properly. In addition, since the sockets in the burn-in board are subject to fatigue from re-use, defective sockets must be detected on each board, and, furthermore, must not be used in subsequent component testing until repaired.
The great length of time associated with the burn-in and testing process (6 to 160 hours) requires that the burn-in chambers be fully utilized to maximize through-put in the process. To maximize through-put, the process should not operate in a sequential step-by-step manner with each burn-in board since doing so would result in underutilization of the burn-in chamber by leaving the chamber unoccupied in whole or in part for significant periods of time Maximum utilization of the chamber thus creates further difficulties in overall integration of the automated burn-in system. This problem is intensified by the requirement that all burn-in boards be loaded into or removed from a single burn-in chamber at the same time since the burn-in boards must be cooled before electrical signals can be removed; once loaded, all boards must remain in the chamber until the burn-in process is completed.
Currently, burn-in boards typically have sockets mounted to one side. Due to the size of the burn-in board, the board warps easily, and fitting the board into the burn-in chambers thus becomes a problem. In addition, board warpage results in deterioration of the burn-in board circuitry. Deterioration of the circuitry is also caused by the insertion and removal of IC packages from the board due to the flexing which occurs to the board during those procedures. Conventional burn-in boards are, additionally, easily damaged and often have chipped-off corners. The trend toward automation will only intensify the above problems.
As a consequence of the problems discussed above and other problems, the industry associated with the burn-in process to date has not achieved an automated, "hands-off" operation.